Recent advances in data processing technology have spurred the development of a number of high speed devices for rendering permanent records of information. Alphanumeric non-impact printing mechanisms now include thermal, electrostatic, magnetic, electrophotograghic, ionic, and, most recently, bubble jet systems. This latter relatively new development is described in detail in the following U.S. Pat. No. 4,243,994 entitled LIQUID RECORDING MEDIUM by Hajime Kobayashi, et al, issued Jan. 6, 1981; U.S. Pat. No. 4,296,421 entitled INK JET RECORDING DEVICE USING THERMAL PROPULSION AND MECHANICAL PRESSURE by Toshitami Hara, et al, issued Oct. 20, 1981; U.S. Pat. No. 4,251,824 entitled LIQUID JET RECORDING METHOD WITH VARIABLE THERMAL VISCOSITY MODULATION by Toshitami Hara, et al, issued Feb. 17, 1981; and U.S. Pat. No. 4,313,124 entitled LIQUID JET RECORDING PROCESS AND LIQUID JET RECORDING HEAD by Toshitami Hara, issued Jan. 26, 1982. Also see copending U.S. patent application Ser. No. 292,841 by John L. Vaught, et al.
In its simplest configuration, the bubble jet printing system consists of a capillary tube containing ink, with one end of the capillary communicating with an ink reservoir and the other end open to permit ejection of an ink droplet. Also included is a resistor either within the capillary or in close proximity to it, providing a sudden burst of thermal energy within the capillary. This burst of energy causes the ink to vaporize in a local region, creating a bubble in the capillary whose sudden expansion creates a pressure wave in the ink and causes an ink droplet or droplets to be expelled from the open end of the capillary.
Although it is not discussed in the above-referenced patents, the best control over the ejection of droplets is obtained when the device is operated in the closed mode, ie. when the bubble is permitted to collapse within the capillary rather than when the ink vapor is permitted to be vented to the outside with the ejection of the droplets. A major problem associated with this closed mode method of printing is that the bubble has a tendency to collapse on or near the resistor, thereby subjecting the resistor to damage each time the bubble collapses. Another difficult problem associated with this method of ink jet printing is that it requires the development of new kinds of inks which can withstand thermal shock without developing significant changes in their physical or chemical composition. Further, the chemical properties of the ink can themselves damage the resistor, especially during bubble collapse. As a result, one of the significant problems in bubble jet technology is resistor lifetime.
To date, typical solutions to the resistor lifetime problems have dealt with protective coatings on the resistor, with special ink formulations which are chemically less damaging to the resistor, and with flexible substrate materials. However, none of the prior art solutions has considered the use of a bubble to drive the ink from the capillary without actually vaporizing the ink.